The synthesis of high quality graphene is one of the hurdles to tackle on the road to graphene devices for advanced technology nodes.
There are several methods known for growing graphene of relatively high quality.
One method consists in annealing crystalline SiC samples to very high temperatures, thereby desorbing Si and triggering the growth of graphene layers at the surface of the wafer (W. De Heer et al., PNAS Oct. 11, 2011 vol. 108 no. 41 16900-16905). One of the limitations of this technique is that one is limited to the size of available crystalline SiC wafers. 5 inch is the largest commercially available size and the prize thereof is prohibitive for many applications.
Similar good results were achieved by growth of graphene by Chemical Vapor Deposition (CVD) on a catalyst metal substrate (K. S. Kim et al., Nature 457, 706-710 (2009)). This kind of approach implies however a transfer of the graphene from a first substrate to a target substrate. This step is difficult and critical.
The quality of the grown graphene is highly influenced by the kind of metal catalysts used as a substrate. Initially people started with metal catalysts similar to these used for the growth of carbon nanotubes (CNTs). Typically however, such catalysts allow too much carbon diffusion therein, thereby preventing good graphene growth control. Most of the reports deal with Cu as the metal catalyst, as Cu can be etched easily without damaging the graphene, which enables an easy transfer procedure. Nevertheless Pt has an intrinsic better potential for the growth of high quality graphene, as it can withstand higher temperatures compared to Cu and there are less nucleation points, resulting in bigger graphene grains.
An important drawback from growing on Pt, is that Pt is difficult to etch and expensive. The cost to deposit Pt and etch it after the growth of one graphene layer is too high for production, especially since the etched Pt cannot be recycled after use for the formation of a second graphene layer.
A solution would be to find a way to remove graphene from the Pt template without harming the Pt layer, allowing for a next graphene growth cycle.
In the literature (L. Gao et al., Nature communications 3, article no 699 (2012)), it was demonstrated that this can be achieved by bubbling method which is non-destructive for the Pt substrate as well as for the graphene. In this method, a graphene film was grown on Pt by CVD. Then the Pt substrate with the graphene grown on it was spin-coated with polymethyl methacrylate (PMMA) followed by curing. The resulting structure is a graphene layer sandwiched between a Pt substrate and a PMMA film. Although most of the interface between the Pt and the graphene is not exposed to the outside world, an interface between the Pt and the graphene is exposed at the edge of this structure. Then the PMMA/graphene/Pt is dipped into a NaOH aqueous solution and used as the cathode of an electrolysis cell. H2 is produced at the cathode, thereby forming bubbles. Some of the bubbles formed at the edge of the PMMA/graphene/Pt structure are formed at the interface between the graphene and the Pt substrate accessible at the edge. These bubbles separate the Pt substrate and the graphene at the edge. This separation progresses from the edges of the PMMA/graphene/Pt toward the centre of the structure, until the graphene/PMMA bilayer is detached from the Pt substrate. However, the graphene film which could so be removed from the Pt was of small dimension (3 cm2) and it is doubtful that this method permits the transfer of larger graphene films without damage. There is therefore a need in the art for another method.